To the surprise of the idea’s originators, Manhattan Borough President Scott Stringer earlier this month unearthed a dusty old proposal for a new subway line that would serve the outer boroughs.

“The X Line would connect all but three subway lines in the city and join Brooklyn, Queens and the Bronx, which no other line does today,” said Stringer, speaking to a room full of New York establishment types at a breakfast hosted by the Association for a Better New York.

“Here’s why it’s not a pipe dream: The line is built entirely along existing rights of way,” said Stringer. “That means no tunneling, which is the biggest hurdle in this day and age to building new subways.”

Stringer’s championing of the X line took some transportation advocates (pleasantly) aback.

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“It was a total surprise,” said Jeff Zupan, a senior fellow for transportation at the Regional Plan Association, which issued the 1996 report in which the proposal, also known as the Triboro RX, was raised. “As a matter of fact, where he learned of it we have no idea.”

The idea goes something like this.

There are, for the most part, existing freight tracks running from Bay Ridge up through Queens and across the Hell Gate Bridge into the Bronx. Freight traffic on those rails is light. And there is, theoretically, enough space alongside them to accommodate some form of commuter rail.

“It doesn’t have to be a subway type car,” said Zupan. “It could be somewhat smaller, but still operate as a train with multiple cars.”

The train would run above ground and intersect with nearly every subway line in the city, avoiding Manhattan altogether.

Aside from the question of financing—no number has been put on just how much this project would cost—there are other potential hang-ups. First, Stringer’s proposal could conflict with his political mentor Rep. Jerry Nadler’s longstanding desire to see the Bay Ridge railyard used as the terminus of a cross-harbor freight tunnel. Further, the Federal Railroad Administration requires a certain distance between freight and passenger trains that share rights of way, and it's not clear whether the existing right of way is, legally, wide enough.

“So the question is whether the F.R.A. criteria can be overcome,” says Zupan.

Though other aspects of the association's 1996 report took root, namely the jumpstarting of the Second Avenue subway and East Side Access, the X line basically vanished.

“Some things stick to the wall and happen and some things don’t,” says Zupan.

In 2008, before the recession rendered discussion of such expansion plans impracticable, former M.T.A. chairman Elliot Sander resurrected the idea in a 40-year plan he presented at the Cooper Union.